Coaxial cables for high frequency signal transmission may be designed for specific operating impedances by adjusting the spacing between the inner conductor and the surrounding outer conductor. To design a coaxial cable for high impedance characteristic, the distance between the inner conductor and the outer conductor is increased and or a dielectric with a higher specific gravity is used. However, application of dielectric materials with higher specific gravities increases the materials cost, weight and signal loss characteristics of the cable. To minimize the overall diameter of a high impedance cable, where high signal power capacity is not a design parameter, the diameter of the inner conductor may be minimized down to that of a fine wire.
A coaxial cable with a fine wire inner conductor, surrounded by a foam dielectric that is covered by the outer conductor presents several manufacturing challenges. A fine wire inner conductor is very fragile. This makes it difficult to smoothly guide the inner conductor with the required precision through a traditional continuous coaxial cable manufacturing process.
Prior high impedance fine wire inner conductor coaxial cables have been observed with an unacceptably high number of longitudinal voids in the dielectric foam, proximate the fine wire inner conductor. These voids introduce variances to the dielectric value of the area between the inner and outer conductor, create a moisture/corrosion path within the cable and also allow the position of the inner conductor within the foam dielectric to vary. Together, these factors introduce a significant error between the designed and the measured characteristic impedance of the finished cable that may vary length to length of the cable.
A prior art coaxial cable with void(s) 5 around the fine wire inner conductor 10, for example as shown in FIG. 1, is difficult to prepare for interconnection because the exact inner conductor position is variable. Also, in contrast to a cable where the inner conductor 10 is fully supported by the foam dielectric 15, any pressure upon the inner conductor 10 during interconnection may cause it to bend and collapse into the void(s) 5, away from the cable end.
Competition within the coaxial cable industry has focused attention upon electrical characteristic uniformity, defect reduction and overall improved manufacturing quality control.
Therefore, it is an object of the invention to provide a coaxial cable and method of manufacture that overcomes deficiencies in such prior art.